1. Field of the Invention
This invention relates to pneumatic tires and more particularly to a novel bead construction for heavy duty pneumatic tires.
2. Prior Art
Generally, heavy duty or high load bearing pneumatic tires include a radial carcass having at least one ply of rubberized steel cord fabric wrapped about a pair of bead cores formed from wires to create carcass flippers or turnups. Bead reinforcement strips are arranged in a folding zone and extend radially outwardly beyond the turnups, and are usually separated therefrom by rubber masses. Positioned above each bead core and extending radially outwardly therefrom between the turnups and the carcass ply is an apex strip of a hard rubber compound.
To guarantee a reliable seating of the tire on a wheel rim which is usually tapered or steep shouldered, i.e., having a seating area sloped at 15.degree. to the wheel axle, it is customary to wind or arrange the wires of the bead core in such a way that the bead core has a polygonal cross-sectional configuration. In order to reduce the manufacturing problems of such tires, it is known in the art to wrap the wires of the bead core with a rubber mass tending to transform the polygonal cross-section into a rounded off overall cross-sectional configuration. Such a construction is disclosed in British Pat. No. 1,163,108, in which the bead core is assembled out of various components and is radially outwardly followed by a standard apex strip of substantially triangular cross-section, whose sides, oriented substantially radially, are considerably longer than its base which is in contact with the wire core. Such a compound bead core design is quite costly.
To be able to build a high load-bearing capacity tire with a bead core of substantially hexagonal cross-sectional configuration on standard flat drum machines, a strip made of hardenable, but not hardened, elastomer may be applied directly onto the radially outwardly situated area of the bead core with a sleeve of loose, rectangular-weave textile material placed around the core and the elastomer strip. The textile material consists preferably of a plastic material subject to shrinkage under the effect of heat. In German Pat. No. 2458423, a semicylindrical elastomer cover is used to supplement the hexagonal cross-section of the bead core so as to form a substantially oval-shaped cross-section. In this case, the transfer of forces from the tire to the rim could be considered to be satisfactory, however, the manufacture of such a bead is complicated and expensive.
Still other bead constructions are known in the art. For example, in U.S. Pat. No. 3,682,223 a bead construction is disclosed having a hard wedge shaped apex strip extending from each bead core and a soft cushion strip disposed between the apex strips and the carcass ply. In U.S. Pat. No. 3,392,773 the bead area is provided with a soft low modulus rubber compound extending from immediately adjacent the bead core to a point radially beyond the edge of an inextensible ply located in the lower sidewall area by using a soft rubber compound as a bead filler and a ply edge gum strip on the edge of the inextensible ply. Other constructions call for elements of various hardness for enveloping the edges of the plies and for acting as bead fillers, e.g., U.S. Pat. No. 3,727,568 and U.S. Pat. No. 3,921,693.
The bead constructions known in the art have the drawback that they are susceptible to premature fatigue cracks in the area of the turnup and/or the bead reinforcement strip, which area constitutes a weak point in the bead. Stability in the bead is unsatisfactory because bead cores and apex strips of known construction will not optimally attenuate or absorb the energy of deformation created in the tire under operating conditions, and because the residual energy of deformation at the turnup or bead reinforcement strip end is excessive and is being dispersed poorly.
It is toward elimination of these and other drawbacks that the present invention is directed.